Intraocular lenses are used as surgically implanted replacements for damaged or diseased natural lenses in the human eye. Lenses of this type include a main lens element or body, and support elements known as haptics which hold the lens in position within the eye. The lens body is typically contoured with convex, concave and/or planar general overall shapes to provide refractive optical power. The contoured areas can have a smooth (i.e., single curve) optical surface. However, complex optical structures are often fabricated on the surfaces of the contoured areas to provide the lens with other optical powers and characteristics. Toric and aspheric lens structures and diffractive and refractive multifocal zone plates are examples of complex surface structures which can be found on intraocular lenses.
Multifocal lenses have several predetermined focal lengths to provide corrections for several ranges of vision (e.g., for driving and reading). These multifocal characteristics are provided by complex optical surface configurations. Lenses with multifocal diffractive zone plates use the principal of diffraction to provide the optical power. The use of multifocal diffractive zone plates in ophthalmic lenses is generally known and disclosed, for example, in the Cohen, U.S. Pat. Nos. 4,210,391, 4,338,005 and 4,340,283. Other lenses have complex surface configurations which utilize the principal of refraction to provide multifocal optical characteristics. Multi-curve multifocal ophthalmic lenses with spherical or aspherical sectors are disclosed in the Nordan U.S. Pat. No. 4,917,681, Nordan U.S. Pat. No. 4,769,033 and Nielson et al. U.S. Pat. No. 4,636,211. A multifocal lens with a spherical upper sector, a spherical middle sector and an aspherical lower sector is shown in the Nordan U.S. Pat. No. 4,917,681. The Shirayanagi U.S. Pat. No. 4,950,057 discloses a lens which includes a refractive multifocal Fresnel surface structure. Other known complex optical surface structures which use multiple zones to provide multifocal optics include the Achatz et al. U.S. Pat. No. 4,813,955, Frieder et al. U.S. Pat. No. 4,869,588, and Frieder et al. U.S. Pat. No. 4,952,048.
Intraocular ophthalmic lenses can be fabricated from a number of different polymer materials including polymethylmethacrylate (PMMA), silicone acrylate, perfluorinated polyethers, and hydrophilic materials such as hydrogels, polyurethanes and silicones. Perspex CQ (clinical grade) and CQ UV (with UV absorber) PMMA manufactured by Imperial Chemical Industries, PLC are especially good ophthalmic lens materials. Because of their extremely high molecular weight (measured in the range of 1.8M to 2.1M using a static low angle laser light scattering method (LALLS)), Perspex CQ and CQ UV offer a high degree of biocompatibility with the human body. Other desirable characteristics of these materials include their hardness, strength and optical quality. These materials are available only in cell-cast sheets.
Imperial Chemical Industries, PLC, the manufacturer of the Perspex PMMA referred to above, recommends that the cell-cast sheets be normalized in recirculating hot air ovens to relieve residual stress in the stock material. Normalization occurs when the cell-cast sheet is heated above its glass transition temperature and cooled under controlled conditions. During normalization, the Perspex sheet will shrink approximately two percent longitudinally and increase proportionately in thickness. However, the normalization treatment may cause depolymerization of the Perspex, and the oxygen can inhibit any re-polymerization, therefore degrading the material.
Intraocular lenses are often machined from lens members such as preformed buttons or semi-finished lens blanks. These buttons and blanks can be machined, sawed or punched from stock polymer material, or individually molded using injection, compression or casting techniques. The semi-finished lens blanks typically have one or more contoured areas. Following the formation of the multifocal diffractive zone plate or other surface structure on the appropriate portion of the lens blank, remaining portions of the blank are machined away. The lens can then be polished using techniques such as tumble polishing to remove any remaining surface roughness.
Mechanical machining methods are typically employed to impart the multifocal diffractive zone plate or other surface structure onto the surface of the lens member. This manufacturing technique has a number of disadvantages. Diamond tool turning of PMMA stock does not yield a satisfactory finish. Subsequent polishing of the machined surface structures is therefore required. However, polishing can detrimentally affect the optical characteristics of the machined surface structures. The optical surface structures are very minute (e.g., several dozen curved echelons 1-5 microns high in a diffractive zone plate), and must be machined to very close tolerances. Expensive equipment requiring skilled operators is therefore needed. These characteristics also make part-to-part reproducibility very difficult and necessitate expensive inspection procedures. Furthermore, it is especially difficult to machine these optical structures on concave or convex contoured surfaces. All of these factors add to the cost of diffractive multifocal ophthalmic lenses.
Intraocular lenses with complex surface structures can also be cast or molded as shown, for example, in the Bissonette et al. U.S. Pat. No. 4,753,653. However, these techniques have a number of disadvantages. Materials used to injection mold lenses are generally less desirable in that they have lower molecular weights and are not as hard. Voids or particulates are often present in molded lenses and exhibit refractiles which degrade the optical characteristics. Injection molded lenses also tend to be less dimensionally faithful to the tool because of warpage caused by variations in the density and location of the mold gate. The fidelity of tool replication can also be compromised by other aspects of the molding process such as venting and ejection.
Intraocular lens haptics can be molded or machined as a one-piece structure along with the optical portion of the lens, as shown in the Bissonette et al. patent referred to above. In other lenses such as those shown in the Chase et al. U.S. Pat. No. 4,242,761, Kaplan et al. U.S. Pat. No. 4,668,446 and Knoll et al. U.S. Pat. No. 4,936,849, the haptics are manufactured as separate members and subsequently attached to the lens. Of course the more time and effort that is required to manufacture and assemble the haptics, the more expensive the resulting intraocular lens.
It is evident that there is a continuing need for improved methods for manufacturing polymer intraocular lenses. Specifically, a relatively inexpensive, efficient and high precision method is needed.